In June 2017, World View (WV) launched a Stratollite (an adjustable altitude, trajectory controlled, lighter-than-air flight vehicle) from Page, AZ to lift a 50 kg commercial payload to the Stratosphere to perform a flight test of the critical Stratollite subsystems. In October 2017, World View (WV) launched the Stratollite from the newly commissioned Tucson Spaceport at WV Headquarters. Using solar rechargeable batteries and a proprietary air ballast system, the Stratollite was able to navigate and loiter over the state of Arizona for 5 days. ~50kg of customer payloads were flown on this mission including a commercial-off-the-shelf (COTS) ,nadir-pointing, 50.6MP Canon EOS 5DS. Post-flight analysis of the Canon images achieved 16cm ground sample distance (GSD) from an altitude of ~75,000ft, which serves as a proof-of-concept for future WV remote sensing geospatial applications. WV is also already involved in several collaborative talks within the imaging community to fly exciting new detector technology and gimbal stabilized cameras.
A Stratocraft (traditionally referred to as a gondola in historic lighter-than-air platforms) is a tetrahedral structure that hangs below the balloon flight train and contains flight avionics, power systems, and a reconfigurable payload deck utilizing a modular open systems approach (MOSA) for customer payloads. Currently, the Stratocraft can host a total mass of 50kg, provide 250W continuous payload power, and 1000W instantaneous payload power, with planned growth to 100kg and 300W continuous by the end of 2018. At the end of flight, the Stratocraft is separated from the flight train, is remotely guided to a specified ground location, and is able perform a flared landing to minimize landing loads to the Stratocraft. This process not only makes the Stratocraft reusable but minimizes risk of damage to payloads.. The Stratocraft has azimuth pointing capability to maintain the vehicle solar arrays pointing at the sun to maximize the efficiency of the solar array. The vehicle pointing resolution and jitter environment is due to be characterized on future flights.
The Stratollite vehicle was designed to operate nominally between the 42nd parallel north and the 42nd parallel south. The vehicle enables long duration (up to 6 months) missions globally with support for launch and flight operations performed from WV headquarters in Tucson, AZ. It is a sensor agnostic long duration capable flight vehicle with long dwell capability depending on regional and seasonal stratospheric wind conditions that offers a multitude of applications to meet the scientific community needs.
This paper outlines development efforts to produce an imaging system, known as the HExapod Resolution Enhancement SYstem (HERESY), that can be interchangeably used aboard balloon-based and ground-based observing platforms. The instrument is a cryogenic hexapod system that accomplishes image stabilization similar to a tip-tilt mirror but by actuating the focal plane rather than the incoming optical beam.
In its balloon configuration, HERESY is not a full-gimbaled pointing instrument but is rather a high-precision, highfrequency image stabilization instrument that removes image blurring caused by jitter. The pointing error signal (collected by star tracker) is fed to the hexapod at a high frequency to drive positional corrections in close to real-time. 1 arcsecond sustained pointing has already been demonstrated by missions such as NASA’s STO-2 Antarctic balloon mission, and HERESY would improve STO-like gondola’s pointing by an order of magnitude (0.1”) bringing the imaging capability of the platform down to the 300nm diffraction limit. These balloon imaging capabilities have caught the interest of the planetary community since a long-duration mission would enable persistent diffraction-limited imaging for applications such as Gas Giant storm tracking, small body remote sensing, and exoplanet detections.
The HERESY instrument is transportable and interchangeable since different detector configurations can be readily interchanged on the hexapod’s mounting surface. HERESY can also be plugged into the focal point of any telescope system without introducing the need for any additional optics of its own. Therefore, it is straight-forward to reconfigure HERESY from a balloon-based instrument to a ground-based instrument. In the ground-based configuration an additional fast-read CMOS detector is co-mounted next to the primary science detector and acts as a star tracker. Once the imaging targets are lined up properly, the CMOS tracks the center-point of the guide-star at a rate of 100-200fps and feeds the positional corrections to the hexapod while the primary detector can take a long exposure simultaneously. Using this technique, the hexapod can remove X-Y blurring error in an image caused by atmospheric turbulence. In this configuration, HERESY can be installed at the focal plane of any optical telescope and immediately provide a working image stabilization system. Engineering testing of this prototype instrument have been completed at the 61” Kuiper observatory in Tucson, AZ, but more refinement of the pointing algorithm is needed before this instrument can collect publishable science data. A known limitation of the instrument is that a bright star must be in the FOV of the CMOS while the science target is in the FOV of the primary detector, so future modifications of the ground-based version of HERESY will likely include the addition of several more fast-read CMOS star trackers to broaden the star tracker field of view.
In this proceeding, we describe the scientific motivation and technical development of the Colorado Highresolution Echelle Stellar Spectrograph (CHESS), focusing on the hardware advancements and testing of components for the fourth and final launch of the payload (CHESS-4). CHESS is a far ultraviolet rocket-borne instrument designed to study the atomic-to-molecular transitions within translucent cloud regions in the interstellar medium. CHESS is an objective echelle spectrograph, which uses a mechanically-ruled echelle and a powered (f/12.4) cross-dispersing grating; it is designed to achieve a resolving power R > 100,000 over the band pass λλ 1000–1600 Å. CHESS-4 utilizes a 40 mm-diameter cross-strip anode readout microchannel plate detector, fabricated by Sensor Sciences LLC, to achieve high spatial resolution with high global count rate capabilities (∼ MHz). An error in the fabrication of the cross disperser limited the achievable resolution on previous launches of the payload to R ∼ 4000. To remedy this for CHESS-4, we physically stress the echelle grating, introducing a shallow toroidal curvature to the surface of the optic. Preliminary laboratory measurements of the resulting spectrum show a factor of 4–5 improvement to the resolving power. Results from final efficiency and reflectivity measurements for the optical components of CHESS-4 are presented, along with the pre-flight laboratory spectra and calibration results. CHESS-4 launched on 17 April 2018 aboard NASA/University of Colorado Boulder sounding rocket mission 36.333 UG. We present flight results for the observation of the γ Ara sightline.
We present a prototype hexapod image stabilization system as the key instrument for a proposed suborbital balloon mission. The unique design thermally isolates an off-the-shelf non-cryogenic hexapod from a liquid nitrogen cooled focal plane, enabling its use in a cryogenic environment. Balloon gondolas currently achieve 1-2 arcsecond pointing error, but cannot correct for unavoidable jitter movements (~20 micron amplitude at 20 Hz at the worst) caused by wind rushing over balloon surfaces, thermal variations, and vibrations from cryocoolers, and reaction wheels. The jitter causes image blur during exposures and limits the resolution of the system. Removal of this final jitter term decreases pointing error by an order of magnitude and allows for true diffraction-limited observation. Tip-tilt pointing systems have been used for these purposes in the past, but require additional optics and introduce multiple reflections. The hexapod system, rather, is compact and can be plugged into the focal point of nearly any configuration. For a 0.8m telescope the improvement in resolution by this system would provide 0.1” angular resolution at 300nm, which is comparable to Hubble for a fraction of the cost. On an actual balloon, the hexapod system would actuate the focal plane to counteract the jitter using position information supplied by guidestar cameras. However, in the lab, we instead simulate guide camera tracking, using a 1024 × 1024 e2v science-grade CCD to take long exposures of a target attached to an XY stage driven with the balloon jitter signal recorded during the STO mission. Further confirmation of the positional accuracy and agility of the hexapod is achieved using a laser and fast-sampling position-sensitive diode. High-resolution time domain multispectral imaging of the gas giants, especially in the UV range, is of particular interest to the planetary community, and a suborbital telescope with the hexapod stabilization in place would provide a wealth of new data. On an Antarctic ~100-day Long-Duration-Balloon (LDB) mission the continued high-resolution imaging of gas giant storm systems would provide cloud formation and evolution data second to only a Flagship orbiter.
Over the past few years there has been remarkable success flying imaging telescope systems suspended from suborbital balloon payload systems. These imaging systems have covered optical, ultraviolet, sub-‐millimeter and
infrared passbands (i.e. BLAST, STO, SBI, Fireball and others). In recognition of these advances NASA is now considering ambitious programs to promote planetary imaging from high altitude at a fraction of the cost of similar fully orbital systems. The challenge with imaging from a balloon payload is delivering the full diffraction-‐limited resolution of the system from a moving payload. Good progress has been made with damping mechanisms and oscillation control to remove most macroscopic movement in the departures of the imaging focal plane from a static configuration, however a jitter component remains that is difficult to remove using external corrections.
This paper reports on work to demonstrate in the laboratory the utility and performance of actuating a detector focal plane (of whatever type) to remove the final jitter terms using an agile hexapod design. The input to this demonstration is the jitter signal generated by the pointing system of a previously flown balloon mission (the Stratospheric Terahertz Observatory, STO). Our group has a mature jitter compensation system that thermally isolates the control head from the focal plane itself. This allows the hexapod to remain at ambient temperature in a vacuum environment with the focal plane cooled to cryogenic temperatures. Our lab design mounts the focal plane on the hexapod in a custom cryostat and delivers an active optical stimulus together with the corresponding jitter signal, using the actuation of the hexapod to correct for the departures from a static, stable configuration. We believe this demonstration will make the case for inclusion of this technological solution in future balloon-‐borne imaging systems requiring ultra-‐high resolution.